Our working hypothesis states that nonreceptor protein-tyrosine kinases transduce environmental signals in fully mature cells. During the current reporting period important evidence has been obtained supporting this hypothesis. In an effort to define possible subcellular locations where the tyrosine kinase activity of p55-c-fgr might be exerted, human PMN was fractionated and assayed for the enzyme. The findings demonstrated that p55-c-fgr is associated with plasma membrane as well as functional secretory granules and is redistributed within normal neutrophils in response to their activation. It was also sought to determine whether platelets, a rich source for p59 association with PI-3 kinase, might serve as a measure of nonreceptor protein-tyrosine kinase activation under physiologic conditions. p60, as well as p59, the product of another member of the src family of proto-oncogenes, was found to physically associate with PI-3 kinase within 5 s after exposure to thrombin. The possible involvement of tyrosine phosphorylation in signalling degranulation through the high-affinity IgE receptor (FcepsilonRI) was examined. Tyrosine phosphorylation was shown to be an early signal following FcepsilonRI aggregation, independent of the exocytotic process itself. These findings functionally link protein phosphorylation on tyrosine residues to FcepsilonRI-mediated signal transduction leading to degranulation.